1. Field of the Invention
The present invention relates to a light receiver employed generally for optical communications and having a relatively wide dynamic range.
2. Description of the Related Art
Conventional light receivers for optical communications include ones of the trans-impedance type shown in FIG. 1 and ones of the high-impedance type shown in FIG. 2. The light receiver shown in FIG. 1 is constructed such that an output terminal of a light receiving element 1 such as a pin-photo diode is connected to an input terminal of a low-noise amplifier 2 (hereinafter, called "front end"). The light receiver of FIG. 2 is constructed such that a bias resistor 3 is connected in series to a light receiving element 1, and a capacitor 4 for preventing a DC is connected between an input terminal of a front end 2 and a node between the light receiving element 1 and the bias resistor 3.
In either structure shown in FIG. 1 or FIG. 2, the circuit is optimized, taking a low light-receiving level into account. When the light-receiving level rises, an electric current or voltage greater than required for optimal operations is applied to the front end 2. In this case, an output waveform is distorted and a correct output waveform cannot be obtained.
Recently, there has been a strong demand for the advent of a light receiver having such a wide dynamic range characteristic as to respond to a high-level light input without fail, thereby matching various transmission distances in an optical communication subscriber system.
This type of light receiver with a wide dynamic range was already made public in lecture No. B-922 (1990) of Electronic Information Communication Association. The technique of this light receiver will now be described in brief with reference to FIG. 3.
This light receiver is an improvement on the trans-impedance type receiver shown in FIG. 1. Referring to FIG. 3, the front end 2 is constituted by a parallel feedback amplifier comprising an FET amplifier 21, a feedback resistor 22 and a DC preventing capacitor 23. A DC portion of an output Vout from the front end 2 is cut off by the capacitor 23, and a resultant signal is fed to a current detector 5. The current detector 5 detects a current value of the input signal. The detected current value is supplied, as a voltage variation, to a difference amplifier 6. The difference amplifier 6 detects a difference voltage between the voltage variation from the current detector 5 and a reference voltage Vref. The difference voltage serves as a source input to an FET 7. The FET 7 biases the output of the light-receiving element 1 in accordance with the level of the source input.
Specifically, if the light-receiving element 1 is DC-coupled to the front end 2, as shown in FIG. 1, the input bias of the front end 2 rises in accordance with the increase in the level of received light. As a result, the output side is saturated and the dynamic range is limited. In the light receiver shown in FIG. 3, the FET 7 is provided on the input side of the front end 2, and the FET 7 is given a function of controlling an input DC level. The current to the FET 7 is increased in accordance with the rise in reception light level. According to this method, a dynamic range of 32 dB can be obtained. There is a problem, however, that the circuit configuration is complex.
The structure of another conventional wide-dynamic range light receiver is disclosed in Published Unexamined Japanese Patent Application (PUJPA) No. 2-226923. This light receiver will now be described in brief with reference to FIG. 4.
This light receiver is a further improvement wherein a capacitor 4 is provided on the input side of a trans-impedance type front end 2 so that the front end 2 is used like the high-impedance type receiver shown in FIG. 2. Specifically, the light receiver has a bias circuit comprising a resistor 8 and a pair of Schottky barrier diodes 9 and 10. A predetermined bias current flows in the bias circuit, thereby setting the output terminal of the light-receiving element 1 at a specified bias level. When the reception light level rises, the impedance of the diode 10 lowers and the impedance of the diode 9 rises. Thus, the current flowing into the front end 2 does not exceed the value of an initial bias current. Accordingly, even if the level of light received rises, output saturation of the front end 2 can be avoided and the wide-dynamic range characteristic attained.
This system, however, always requires a bias current, and power consumption is high. At a minimum reception light level, the impedances of the diodes 9 and 10 are equal and the current is equally divided. Consequently, the minimum reception light level is degraded by about 3 dB.
As has been described above, in the conventional wide-dynamic range light receivers, the circuit configuration is complex and, even if the dynamic range is expanded to a high reception light level, the minimum reception light level is degraded.